This invention relates to blow molding of oriented hollow articles of thermoplastic material and more specifically to an oven for heating parisons to a predetermined temperature.
In recent years, there has developed techniques whereby blow molded articles are produced having biaxial orientation, which articles have exceptional strength and highly desirable physical properties such as clarity. This technique involves forming a parison, cooling it to well below its melting point, and thereafter heating it to its orientation temperature. The orientation temperature is just below the crystalline melt point in the case of crystalline materials and 40.degree. to 225.degree. F. below the homogeneous melt point for amorphous materials. Such techniques are disclosed in U.S. Pat. Nos. 3,288,317 and 3,390,426.
Ovens for heating such parisons can be found in U.S. Pat. Nos. 3,752,641; 3,740,868; and 3,801,623, and in co-pending application U.S. Ser. No. 473,290, filed May 24, 1974, all of the above assigned to the assignee of the present invention. Generally, tubular parisons are carried on pin structures on a movable chain through the oven. As will be readily appreciated from a review of the prior art cited above, the two major problems relating to the oven are: the time it takes to heat certain materials, such as polypropylene, and uniformly heating the inner and outer parison walls along the axial length of the parison. In U.S. Ser. No. 473,290, said application hereby being incorporated by reference, the velocity of the heating fluid is increased to the distortion velocity of the parisons, and a tempering zone was added to reduce the heating time to equalize the temperatures of the inner and outer walls of the parisons. But, the prior art ovens do not provide for uniformly heating parisons which have various axial lengths. The oven was designed to heat the parisons of a predetermined maximum length, but in doing so, when parisons of a predetermined minimum length are heated, large stray currents occur which result in a nonuniform heating environment. The resultant bottles of the smaller lengths are often unsatisfactory.
What is desired then is a parison oven which can minimize the stray heating air currents within the oven and provide a uniform temperature profile to parisons of various axial lengths to uniformly heat the same.